Print wheel positioning system

ABSTRACT

A print wheel positioning system. A stepping circuit is associated with each print wheel, and a common step/sense line is extended to each stepping circuit. The step/sense line is normally grounded. In order to advance the print wheels by one position, the step/sense line is ungrounded; this causes each stepping circuit associated with an improperly positioned print wheel to advance the print wheel and to apply a positive potential to the step/sense line. As soon as a wheel is positioned correctly, the ungrounding of the step/sense line has no effect on the respective stepping circuit. As long as at least one print wheel has not yet been properly positioned, the step/sense line goes positive each time it is ungrounded. As soon as all print wheels are properly positioned, the step/sense line does not go positive when it is ungrounded and a print command is generated.

United States Patent [72] Inventor Martin Raphael New York, N.Y. [21] Appl. No. 883,219 [22] Filed Dec. 8, 1969 [45] Patented Oct. 19, 1971 [73] Assignee Dytro Corporation I-licksville, N.Y.

[5 4] PRINT WHEEL POSITIONING SYSTEM 14 Claims, 2 Drawing Figs. [52] US. Cl 318/625, 318/685, 101/212 [51] Int. Cl G05b 11/32 [50] Field of Search 318/625, 603, 685, 487; 101/212 [56] References Cited UNITED STATES PATENTS 2,717,980 9/1955 Taylor et al 318/625 2,972,093 2/1961 Silhavy 3,399,753 9/1968 Revelle 3,454,855 7/1969-Morlen,

Primary Examiner-Benjamin Dobeck Att0rney-Gottlieb, Rackman & Reisman ABSTRACT: A print wheel positioning system. A stepping circuit is associated with each print wheel, and a common step/sense line is extended to each stepping circuit. The step/sense line is normally grounded. In order to advance the print wheels by one position, the step/sense line is ungrounded; this causes each stepping circuit associated with an improperly positioned print wheel to advance the print wheel and to apply a positive potential to the step/sense line. As soon as a wheel is positioned correctly, the ungrounding of the step/sense line has no effect on the respective stepping circuit. As long as at least one print wheel has not yet been properly positioned, the step/sense line goes positive each time it is ungrounded. As soon as all print wheels are properly positioned, the step/sense line does not go positive when it is ungrounded and a print command is generated.

POSITIONING ALARH l l CONTROL PATENTEDBCI 19 IHYI 4, 7

sum 1 or 2 POSITIONING comm POSITIONING CONTROL v 1cm '1' (m HA II RAPHAEL PRINT WHEEL POSITIONING SYSTEM This invention relates to printer mechanisms, and more particularly to print wheel positioning systems.

In a typical printer mechanism, a series of print wheels is provided each of which has a series of raised characters (e.g., numerals or letters of the alphabet) around its periphery. The print wheels are turned together and each wheel is stopped when the character to be printed in the respective position is facing the paper. After all the wheels have been properly positioned, a print bar is actuated to control the printout.

The various shortcomings of prior art-printing mechanisms can be best understood by considering a typical system. (It is to be understood that not all of the prior art systems are of the same type, and that the system described is merely illustrative.) At the start of the printing cycle, all of the print wheels are reset to their initial positions. The wheels than all start to turn, with each wheel stopping when its angular position corresponds to the respective character in the data to be printed. In the case of a wheel having ten characters, ten commutator segments may be provided together with a wiper contact which rotates with the print wheel. Depending on the character to be printed, one of the commutator segments is energized. As the wheel rotates the wiper contact passes over the various commutator segments. As soon as the wiper contact engages the energized segment, the potential extended through the contact causes movement of the print wheel to cease. The print wheel remains in the selected position until the end of the print wheel positioning sequence, at which time the actual printing takes place.

However, it is possible for a print wheel to be positioned incorrectly. For example, if for some reason (e.g., contact jitter) the wiper contact does not make physical contact with the energized commutator segment during rotation of the print wheel, the print wheel will continue to rotate all the way around to the last position, and the wrong character will be printed. Prior art print wheel positioning systems in general have not provided a mechanism for verifying prior to printing that each print wheel has indeed assumed its proper position.

It is a general object of my invention to provide a wheel positioning system in which the proper positioning of all wheels is verified prior to the generation of a subsequent command.

It is another object of my invention to utilize the same circuitry, and in particular the same bus, both for controlling the stepping of all wheels and for verifying that all of the wheels are properly positioned.

Although in a typical prior art system each print wheel is stopped when it reaches its proper position, in general the mechanical linkages for moving each print wheel do not come to rest. Instead, it is only the print wheel which stops moving. For example, while the linkages may continue to move, a clutch mechanism may be provided to lock the print wheel in place after it is properly positioned even though the remaining mechanical elements continue to move. This kind of arrangement obviously causes a maximum amount of wear of the moving parts.

It is another object of my invention to provide a print wheel positioning system ,in which all mechanical movements associated with each print wheel cease as soon as the wheel is properly positioned even though the mechanical elements associated with the other print wheels may continue to move.

As described above, in the case of a mechanical or electrical failure it is possible for a print wheel to be advanced past its proper position. While in the prior art this generally results in the printing of an erroneous character, in accordance with the principles of my invention an erroneous character is not printed. Nevertheless, rather than to simply ascertain the fact that all of the print wheels are not positioned properly, it is preferable to attempt to reposition the improperly positioned wheels. This is not possible in the prior art where a maximum of only one rotation of each wheel is possible and once the proper position has been passed it cannot be reached once again as the wheel continues to turn.

It is another object of my invention to provide a print wheel positioning system in which each print wheel is given at least one extra chance" to properly position itself in the event it did not do so on its first try.

On the other hand, consider the more common case in which all of the print wheels do properly position themselves. In the prior art, before the actual print command is generated a sufficient time is allowed for all, of the wheels to rotate to their furthest positions. In other words, if the starting position of each wheel is the numeral 0 and the terminal position of each wheel is the numeral 9, even if all wheels are to be positioned at the numeral 2, the print command is not generated until after the wheels have had a sufficient time interval to move to their terminal positions. In high-speed applications it would be more advantageous to generate a print command immediately upon the proper positioning of the print wheels.

It is another object of my invention to provide a print wheel positioning system in which the printing operation takes place immediately upon the proper positioning of all print wheels.

In many applications, print wheels are often added as time goes on and the system is modified to collect and print greater quantities of data. Furthermore, not all of the print wheels are necessarily used to print all of the available characters. For example, one print wheel might be used only to print either plus or minus signs.

It is another object of my invention to provide a printwheel positioning system with a verification capability which is also highly flexible, that is, in which additional positioning wheels can be added without modifying the basic verification circuitry and the additional circuitry required for each print wheel is reduced in complexity asthe possible characters which can be printed are reduced in number.

Briefly, in accordance with the principles of my invention, the position of each print wheel is incremented when an as: sociated stepping transistor is pulsed on. A single step/sense line is provided and this line is coupled to all of the transistor stepping circuits. Also coupled to thecommon step/sense line is a wiper contact associated with each print wheel. The commutator segments associated with each print wheel are coupled to input transistors, only one of which is in a statedifferent from the others for representing the character to be printed.

The step/sense line is normally grounded. Each time the print wheels are to advance one step, the step/sense line is ungrounded. If any print wheel is in an incorrect position, the associated stepping transistor is pulsed when the step/sense line is ungrounded and the print wheel advances one step. The stepping transistor circuit also causes a positive pulse to appear on the ungrounded step/sense line.

After any print wheel reaches its correct position, a potential extended from the associated wiper contact prevents the associated stepping transistor from turning on during successive cycles as the step/sense line is ungrounded. The stepping transistor circuit also has no efi'ect on the potential of the step/sense line. As long as at least one print wheel is in an incorrect position, the associated stepping transistor turns on each time the step/sense line is ungrounded, and the step/sense line goes positive. Only after all print wheels are properly positioned does the step/sense line not go positive when it is ungrounded. This is an indication that all the wheels are properly positioned and that the printing operation can take place.

In this manner, before a print command is generated the proper positioning of all print wheels is verified a print command is generated only if the step/sense line does not go positive when it is ungrounded. The use of a single common bus coupled to all of the stepping transistors allows for great flexibility and the addition of new print wheels to the system without requiring major circuit changes. The mechanical linkages associated with each print wheel cease to operate the moment the print wheel is properly positioned -the associated stepping transistor, which controls the linkages, does not turn on with the ungrounding of the step/sense line after the print wheel is properly positioned. And the print command is generated immediately upon the proper positioning of all print wheels without requiring the completion of the maximum duration stepping sequence.

In the event a print wheel has accidentally skipped through its proper position, the step/sense line continues to be ungrounded a number of times greater than the number of positions of any print wheel. This allows a print wheel to become positioned properly during a second rotation. Similarly, in the event a print wheel has not stepped as many times as the step/sense line has been ungrounded, the additional ungroundings of the line allow the wheel to catch up to where it should be.

The number of extra chances allowed for a print wheel to become properly positioned is determined by the maximum number of times that the step/sense line is allowed to become ungrounded during each stepping sequence.

Further objects, features and advantages of my invention will become apparent upon consideration of the following detailed description in conjunction with the drawing, in which:

FIGS. 1 and 2, with FIG. 1 being placed to the left of FIG. 2, depict an illustrative embodiment of my invention.

Printer 10, shown only schematically, contains a number of print wheels PWl-PWN. Each print wheel is turned by a respective one of shafts SHl-SI-IN, and a respective one of wiper contacts WCl-WCN is associated with each print wheel.

Each wiper contact is connected to a respective one of conductors Sl-SN, each conductor being included in one of cables K-l through K-N. Associated with each print wheel are ten commutator segments. For example, associated with print wheel PWI are commutator segments 1-0 through 1-9. Each of the commutator segments is connected over a respective conductor (such as conductors CI- through Cl-9 in cable K-l) to a respective one of positioning control circuits PCl-PCN.

Data input source 28 is connected by a plurality of cables Il-IN to respective ones of the positioning control circuits. Each input cable includes a number of conductors coupled to the base terminals of respective transistors in the associated positioning control circuit. For example, cable Il includes ten conductors connected to the base terminals of transistors l-TO through lT9 in positioning control circuit PCl. Nine of the ten input conductors are low in potential to maintain the associated nine transistors in a cutoff condition. The tenth input conductor, representing the required position of the respective print wheel, is high in potential to enable conduction of the associated transistor. However, it should be noted that the associated transistor does not conduct simply because the base terminal is positive in potential. The transistor can conduct only if a positive potential is applied to its collector conductor (Cl-0 through Cl-9).

The collector of transistor 38 serves as a common step/sense terminal, connected to various branches of a common step/sense line. Ordinarily, the transistor is conducting; current flows from source 32 through resistors 34 and 36, the base of the transistor is held at a positive potential and the base-emitter junction is forward biased. The collector of the transistor is shorted through the transistor to ground. In positioning control circuit PCl, current flows from source 12 through resistor 14 and diode DI, the current then flowing through the collector-emitter circuit of transistor 38 to ground. A similar current flows from each of the other positioning control circuits; each of diodes D2 through DN is connected to the collector of transistor 38.

Pulse generator 40 supplies negative pulses to the base of transistor 38 at a rate of 40 per second in the illustrative embodiment of the invention. Each pulse reverse biases the baseemitter junction of transistor 38 and the collector of the transistor becomes ungrounded. Referring to positioning control circuit PC 1, and assuming that conductor Sl has no efi'ect on the circuit, it is seen that while diode D1 conducts transistor 24 is held off since the anode of diode D1 is connected through the diode and transistor 38 to ground. However, as soon as the ground connection is removed from the cathode of diode D1, its anode and the anode of diode 16 rise in potential. Current flows from source 12 through resistor 14, diode l6, resistor 22 and the base-emitter junction of transistor 24. The transistor conducts and current flows through the collector circuit including element 26. Element 26 is shown only symbolically and represents, for example, a solenoid which when pushed causes shaft SI-Il to rotate to the next position. Resistor 18 and diode 20 are provided to limit reverse voltages across element 26 when transistor 24 turns off as is known in the art.

In FIG. 2, wiper contact WC-l is shown engaging commutator segment ll. Suppose that print wheel PWI is to be positioned at an angle corresponding to commutator segment 1-4. In such a case, the input conductor in cable ll connected to the base of transistor l-Tl is low in potential. Conductor Cll is connected cable K-l to the collector of transistor I-Tl. The conductor is not extended through transistor l-Tl to ground since the base-emitter junction of the transistor is reverse biased. Consequently, conductor S1, connected to wiper contact WC-l, is not grounded and has no efi'ect on the conduction of diodes D1 and 16. When a negative pulse is applied to the base of transistor 38, the collector terminal is ungrounded. At this time the potential of source 12 is extended through diode D1 to cause the collector terminal of transistor 38 to go positive. At the same time, transistor 24 conducts to advance shaft SHI one step.

Each time transistor 38 is turned off by pulse generator 40, the positive potential of source 12 is extended through diode D1 to cause the collector of the transistor to go high and transistor 24 to conduct to step print wheel PWl.

Eventually, however, wiper contact WCl engages commutator segment [-4. This commutator segment is connected over conductor Cl-4 to the collector of transistor l-T4 (not shown), the transistor whose base terminal is held at a high potential by the data input source. At this time, current flows from source 12, through resistor 14, conductor 8!, wiper contact WC-l, commutator segment l-4, conductor Cl-4 and the collector-emitter circuit of transistor l-T4 to ground. With the transistor conducting, the anodes of diodes DI and 16 are shorted to ground through the transistor. Transistor 24 cannot turn on since its base-emitter junction is not forward biased. Transistor 24 ceases to turn on with the application of pulses to the base of transistor 38 and print wheel PWI remains in the same position.

Until print wheel PWl is in the proper position, each time a negative pulse is applied to the base of transistor 38, a positive potential is extended from source 12 through diode D1 to the step/sense terminal at the collector of the transistor. As soon as print wheel PWI is properly positioned, however, a positive potential is no longer extended through diode D1 to the step/sense terminal since the anode of the diode is grounded through the conducting one of transistors I-T0 through l-T9. Positioning control circuit PCl does not ground the collector terminal of transistor 38. Instead, it has no effect on the potential of the collector terminal since diode D1 is reversed biased assuming that some other print wheel is not yet properly positioned and a positive potential is extended through the respective one of diodes D2-DN to the step/sense terminal.

As long as any print wheels are improperly positioned, when the collector terminal of transistor 38 is ungrounded a positive potential will be extended through the respective ones of diodes Dl-DN to the step/sense terminal and a positive pulse will appear at this terminal. At the same time, the associated print wheels will be advanced one step. It is only when all print wheels are properly positioned that all of diodes DI-DN have their anodes grounded. At this time, the step/sense line does not go positive with the application of a negative pulse to the base of transistor 38.

It should be noted that even when transistor 38 conducts (between the application of negative pulses at its base terminal), there is is a 0.5-volt drop across each of diodes Dl-DN since the cathode of each diode is connected to the grounded collector terminal of transistor 38 and each diode has a 0.5 -volt drop across it when it is conducting. Ordinarily, this 0.5 -volt drop might be sufl'rcient to turn on the associated stepping transistor such as transistor 24 in positioning control PC]. Diode 16 is provided to prevent conduction of the transistor. The diode, also having a 0.5 -volt forward drop, in efiect requires a potential greater than 0.5 volts at the anode of diode D1 before transistor 24 can turn on.

After data input source 28 energizes the various conductors in cables Tl-lN, it applies a pulse to the set input of flip-flop 30. The 1 output of the flip-flop goes high to enable pulse generator 40. The pulse generator applies negative pulses 70 at a 40-per-second rate to the base of transistor 38. Each pulse ungrounds the collector of the transistor. By the term ungrounds" is meant that transistor 38 simply presents an open circuit to the various conductors connected to the collector terminal. The collector terminal is initially grounded, and the ground connection is removed with the application of a pulse 70 to the base of the transistor. Whether or not the collector of the transistor (step/sense line) actually goes positive when transistor 38 turns ofi depends on whether a positive potential is extended to the common bus through one of the diodes Dl-DN.

The collector of transistor 38 is extended over conductor 65 to the junction of resistors 50 and 52. As long as at least one print wheel is improperly positioned, a positive pulse 72 appears at the resistor junction when transistor 38 turns ofi. Resistor 52 and capacitor 54 form an integrator, the voltage across capacitor 54 rising and then falling exponentially as shown by waveform 73.

Pulse generator 40 also supplies positive pulses 71 on conductor 42, in phase with negative pulses 70. The positive pulses are differentiated by the differentiator including capacitor 58 and resistor 56 to fonn two spikes 74 and 75. These spikes are added to the expontial wavefonn 73 at the junction of resistor 56 and capacitor 58.

As shown by the waveforms adjacent this junction, there are two possibilities. The first is where at least one of the print wheels is improperly positioned and the step/sense line rises in potential. ln-such a case, positive spike 74 and negative spike 75 are superimposed on wavefonn 73. At no time does the resulting potential drop below ground level. Following the superimposed waveforms is shown the situation in which all of the print wheels are properly positioned. in such a case, when transistor 38 turns off, the step/sense line does not rise in potential and waveform 73 does not appear across capacitor 54. All that remains are the positive and negative spikes 74 and 75. Diode 60 blocks the positive spike but allows the negative spike to be transmitted through it. The resulting negative spike across resistor 62 appears on print command conductor 64 to notify the printer to print out the data, that is, to notify the printer that all of the print wheels have been positioned properly.

The resulting negative spike on conductor 64 is also extended to the reset inputs of flip-flop 30 and counter 44. When the flip-flop is reset, the 1 output goes low in potential and pulse generator 40 turns off.

While the pulse generator is operating, the positive pulses 71 on conductor 42 are applied to the input of counter 44. The counter increments its count until a maximum count of 20 is reached. At this time, decode 20" circuit 46 detects a count of 20 in the counter and operates an alarm circuit 48. The various print wheel are given a maximum of two rotations to properly position themselves. (A built-in delay in the counter input allows the counter to be reset if the last wheel is properly positioned on the twentieth pulse prior to the incrementing of the counter to a count of 20.) Although in the usual case each wheel will be properly positioned after a maximum of ten pulses 70 have been applied to the base of transistor 38, it is possible that one or more print wheels will not properly position themselves on the first turn. A maximum of 20 operations of transistor 38 are allowed in order to give each print wheel a second chance" to properly position itself. Of course, the maximum number of pulses can be extended to 30, 40, etc., depending on the particular application, the maximum time available for printing, etc.

It is apparent that the step/sense line functions in a double capacity. Ordinarily, the line is grounded to prevent the turning on of the stepping transistor in each of the positioning control circuits. Each negative pulse 70 ungrounds the step/sense line by turning off transistor 38. The ungrounding of the line does not necessarily cause all of the stepping transistors to turn on. Each stepping transistor turns on only if the associated wiper contact is not in engagement with the selected commutator segment. Each positioning control circuit which causes its associated print wheel to advance also causes the step/sense line to go positive in potential. When all of the print wheels are in their proper positions, none of the positioning control circuits causes the step/sense line to go positive. If the step/sense line does not go positive when it is ungrounded, it is an indication that all of the print wheels are in their proper positions and that the print sequence should commence.

A print command is generated not after ten stepping pulses have occurred, but as soon as all print wheels are properly positioned. It is not necessary to 'go through a complete stepping cycle before a print command is generated. The print command is generated immediately as soon as all of the print wheels are properly positioned.

The system also offers great flexibility. If at any time it becomes necessary to add a print wheel in the printer, the associated positioning control circuit can be tied to the step/sense line simply through a diode similar to one of the diodes Dl-DN. The step/sense line functions with any number of positioning control circuits. Moreover, suppose that a particular print wheel is only to print a limited number of characters. In such a case, it is not necessary to use ten transistors such as l-TO through l-T9 in positioning control circuit PC 1, all ten conductors in the associated input cable, or all ten conductors connecting the ten commutator segments to the transistor collectors. if any position of the print wheel is not to be selected, the associated input conductor, transistor and commutator segment can be omitted. This is due to the fact that an unselected position causes the stepping transistor, such as transistor 24 in positioning control circuit PCI, to be pulsed on because the associated wiper contact engages the collector of a transistor which is held off. if a particular position is to be skipped over, there is no reason to provide the transistor and the associated connections in the first place -when the wiper contact is in the respective position the absence of a connection to an enabled transistor will allow the associated stepping transistor to be pulsed on. The arrangement of the invention thus allows stages to be added with a minimum of wiring complexity.

Although the invention has been described with reference to a particular embodiment, it is to be understood that this embodiment is merely illustrative of the application of the principles of the invention. For example, the same control system can be used with display wheels with no print capability or with other types of electromechanical stepping devices. Numerous modifications may be made in the illustrative embodiment of the invention and other arrangements may be devised without departing from the spirit and the scope of the invention.

What] claim is:

l. A wheel-positioning system comprising a plurality of wheels, a plurality of means each for representing the desired position of a respective one of said wheels, stepping means associated with each of said wheels for advancing said wheel, means for energizing all of said wheel stepping means, means for preventing the energization of each of said wheel stepping means when the respective wheel is in the position represented by the respective representing means, and means responsive to the prevention of the energization of all of said wheel stepping means when said energizing means is operated for indicating that all of said wheels have been positioned in the positions represented by said plurality of representing means.

2. A wheel-positioning system in accordance with claim 1 wherein each of said wheels is a print wheel and further including means for controlling a printing operation, and means for operating said controlling means the first time said energizing means is operated when said indicating means indicates that all of said print wheels have been positioned in the positions represented by said plurality of representing means.

3. A wheel-positioning system in accordance with claim 2 further including a common bus coupled to all of said wheel stepping means, said common bus normally being maintained at a first potential, said energizing means including means for removing said first potential from said common bus, each of said wheel stepping means including means for applying a second potential to said common bus responsive to the removal of said first potential therefrom if the respective print wheel is not in the position represented by the respective representing means, and said indicating means including means for detecting the absence of said second potential on said common bus when said first potential is removed therefrom.

4. A wheel-positioning system in accordance with claim 1 further including a common bus coupled to all of said wheel stepping means, said energizing means including means for maintaining said common bus at a first potential and for removing said first potential in order to energize said wheel stepping means, each of said wheel stepping means including means for applying -a second potential to said common bus responsive to the energization thereof, said indicating means including means for detecting the absence of said second potential on said common bus.

5. A wheel-positioning system in accordance with claim 4 wherein at least one of said wheels has N positions and said energizing means is operative up to at least 2N times responsive to the representing in said representing means of the desired positions of said wheels.

6. A wheel-positioning system in accordance with claim 1 wherein at least one of said wheels has N positions and said energizing means is operative up to at least 2N times responsive to the representing in said representing means of the desired positions of said wheels.

7. A wheel-positioning system in accordance with claim 1 wherein said energizing means includes a common bus normally held at ground potential; said preventing means associated with each of said wheel stepping means includes a normally ungrounded conductor and means for grounding said conductor when the associated wheel is in the position represented by the respective representing means; and each of said wheel stepping means includes a potential source, a pulsing circuit and a diode, said diode connecting said conductor, said potential source and said pulsing circuit to said common bus such that when said conductor is ungrounded said diode is forward biased to allow said potential source to operate said pulsing means and to energize said common bus and when said conductor is grounded said diode is reverse biased to prevent said potential source form operating said pulsing means and energizing said common bus, said pulsing circuit being operative to advance the associated wheel.

8. A wheel-positioning system comprising a plurality of wheels, a plurality of means each for representing the desired position of a respective one of said wheels, stepping means associated with each of said wheels for advancing said wheel, means for energizing those of said wheel stepping means whose respective wheels are in positions other than those represented by the respective representing means, and means for detecting that all of said wheels have been positioned in the positions represented by said plurality of representing means.

9. A wheel-positioning system in accordance with claim 8 wherein each of said wheels is a print wheel and further including means for controlling a printing operation, and means for operating said controlling means immediately responsive to the operation of said detecting means.

10. A wheel-posltionmg system in accordance with claim 9 further including a common bus coupled to all of said wheel stepping means, said common bus normally being maintained at a first potential, said energizing means including means for removing said first potential from said common bus, each of said wheel stepping means including means for applying a second potential to said common bus responsive to the removal of said first potential therefrom if the respective print wheel is not in the position represented by the respective representing means, and said detecting means includes means for detecting the absence of said second potential on said common bus when said first potential is removed therefrom.

11. A wheel-positioning system in accordance with claim 8 further including a common bus coupled to all of said wheel stepping means, said energizing means including means for maintaining said common bus at a first potential and for removing said first potential in order to energize said wheel stepping means, each of said wheel stepping means including means for applying a second potential to said common bus responsive to the energization thereof, said detecting means including means for detecting the absence of said second potential on said common bus.

12. A wheel-positioning system in accordance with claim 11 wherein at least one of said wheels has N positions and said energizing means is operative up to at least 2N times responsive to the representing in said representing means of the desired positions of said wheels.

13. A wheel-positioning system in accordance with claim 8 wherein at least one of said wheels has N positions and said energizing means is operative up to at least 2N times responsive to the representing in said representing means of the desired positions of said wheels.

14. A wheel-positioning system in accordance with claim 8 wherein said energizing means includes a common bus normally held at ground potential; and each of said wheelstepping means includes a normally ungrounded conductor, means for grounding said conductor when the associated wheel is in the position represented by the respective representing means, a potential source, a pulsing circuit and a diode, said diode connecting said conductor, said potential source and said pulsing circuit to said common bus such that when said conductor is ungrounded said diode is forward biased to allow said potential source to operate said pulsing means and to energize said common bus and when said conductor is grounded said diode is reverse biased to prevent said potential source from operating said pulsing means and energizing said common bus, said pulsing circuit being operative to advance the associated wheel. 

1. A wheel-positioning system comprising a plurality of wheels, a plurality of means each for representing the desired position of a respective one of said wheels, stepping means associated with each of said wheels for advancing said wheel, means for energizing all of said wheel stepping means, means for preventing the energization of each of said wheel stepping means when the respective wheel is in the position represented by the respective representing means, and means responsive to the prevention of the energization of all of said wheel stepping means when said energizing means is operated for indicating that all of said wheels have been positioned in the positions represented by said plurality of representing means.
 2. A wheel-positioning system in accordance with claim 1 wherein each of said wheels is a print wheel and further including means for controlling a printing operation, and means for operating said controlling means the first time said energizing means is operated when said indicating means indicates that all of said print wheels have been positioned in the positions represented by said plurality of representing means.
 3. A wheel-positioning system in accordance with claim 2 further including a common bus coupled to all of said wheel stepping means, said common bus normally being maintained at a fIrst potential, said energizing means including means for removing said first potential from said common bus, each of said wheel stepping means including means for applying a second potential to said common bus responsive to the removal of said first potential therefrom if the respective print wheel is not in the position represented by the respective representing means, and said indicating means including means for detecting the absence of said second potential on said common bus when said first potential is removed therefrom.
 4. A wheel-positioning system in accordance with claim 1 further including a common bus coupled to all of said wheel stepping means, said energizing means including means for maintaining said common bus at a first potential and for removing said first potential in order to energize said wheel stepping means, each of said wheel stepping means including means for applying a second potential to said common bus responsive to the energization thereof, said indicating means including means for detecting the absence of said second potential on said common bus.
 5. A wheel-positioning system in accordance with claim 4 wherein at least one of said wheels has N positions and said energizing means is operative up to at least 2N times responsive to the representing in said representing means of the desired positions of said wheels.
 6. A wheel-positioning system in accordance with claim 1 wherein at least one of said wheels has N positions and said energizing means is operative up to at least 2N times responsive to the representing in said representing means of the desired positions of said wheels.
 7. A wheel-positioning system in accordance with claim 1 wherein said energizing means includes a common bus normally held at ground potential; said preventing means associated with each of said wheel stepping means includes a normally ungrounded conductor and means for grounding said conductor when the associated wheel is in the position represented by the respective representing means; and each of said wheel stepping means includes a potential source, a pulsing circuit and a diode, said diode connecting said conductor, said potential source and said pulsing circuit to said common bus such that when said conductor is ungrounded said diode is forward biased to allow said potential source to operate said pulsing means and to energize said common bus and when said conductor is grounded said diode is reverse biased to prevent said potential source form operating said pulsing means and energizing said common bus, said pulsing circuit being operative to advance the associated wheel.
 8. A wheel-positioning system comprising a plurality of wheels, a plurality of means each for representing the desired position of a respective one of said wheels, stepping means associated with each of said wheels for advancing said wheel, means for energizing those of said wheel stepping means whose respective wheels are in positions other than those represented by the respective representing means, and means for detecting that all of said wheels have been positioned in the positions represented by said plurality of representing means.
 9. A wheel-positioning system in accordance with claim 8 wherein each of said wheels is a print wheel and further including means for controlling a printing operation, and means for operating said controlling means immediately responsive to the operation of said detecting means.
 10. A wheel-positioning system in accordance with claim 9 further including a common bus coupled to all of said wheel stepping means, said common bus normally being maintained at a first potential, said energizing means including means for removing said first potential from said common bus, each of said wheel stepping means including means for applying a second potential to said common bus responsive to the removal of said first potential therefrom if the respective print wheel is not in the position represented by the respective representing means, and said detecting means includes means for detecting the absence of said second potential on said common bus when said first potential is removed therefrom.
 11. A wheel-positioning system in accordance with claim 8 further including a common bus coupled to all of said wheel stepping means, said energizing means including means for maintaining said common bus at a first potential and for removing said first potential in order to energize said wheel stepping means, each of said wheel stepping means including means for applying a second potential to said common bus responsive to the energization thereof, said detecting means including means for detecting the absence of said second potential on said common bus.
 12. A wheel-positioning system in accordance with claim 11 wherein at least one of said wheels has N positions and said energizing means is operative up to at least 2N times responsive to the representing in said representing means of the desired positions of said wheels.
 13. A wheel-positioning system in accordance with claim 8 wherein at least one of said wheels has N positions and said energizing means is operative up to at least 2N times responsive to the representing in said representing means of the desired positions of said wheels.
 14. A wheel-positioning system in accordance with claim 8 wherein said energizing means includes a common bus normally held at ground potential; and each of said wheel-stepping means includes a normally ungrounded conductor, means for grounding said conductor when the associated wheel is in the position represented by the respective representing means, a potential source, a pulsing circuit and a diode, said diode connecting said conductor, said potential source and said pulsing circuit to said common bus such that when said conductor is ungrounded said diode is forward biased to allow said potential source to operate said pulsing means and to energize said common bus and when said conductor is grounded said diode is reverse biased to prevent said potential source from operating said pulsing means and energizing said common bus, said pulsing circuit being operative to advance the associated wheel. 